Since we don't have a clue about any of the sizes of trees or flora, since none of those species presumably survived to the present day, the only concievable item we could use for comparison is the Plant Cell shown at the beginning of LBT 11. Since most plant cells are about the same size regardless of the era, someone with way too much time could figure out the size of the entire plant, compensating for the zooming out, and then compare Littlefoot to the size of the plant.
So yeah. Good luck with that.